Wes Duenkel
May 30, 2012

We didn't used to think about bumpsteer--until our newly lowered Mustang would almost change lanes when hitting a bump. Correcting it involved guesswork and complex tools. That's not so anymore.

Bumpsteer is actually a change in the toe caused by the front end going up and down, and the result is a front end that doesn't track in the direction you are commanding. This ill-handling condition can make the car seem unpredictable and unsafe.

Maximum Motorsports' bumpsteer kits and handy bumpsteer gauge make correcting bumpsteer relatively easy and straightforward. It's also recommended to correct rear bumpsteer when lowering an independent rear suspension, such as in our '03 Cobra.

After setting the front and rear toe settings to zero, we used Maximum Motorsports' bumpsteer gauge to adjust the front and rear bumpsteer curves. This minimized toe changes when driving to maintain directional stability and handling consistency. We were pleased to find the gauge accurate, precise, and intuitive to use, and were rewarded with a Mustang that tracks straight and true--even when the pavement isn't.

Definitions

  • Bumpsteer: When a tire's toe setting changes during vertical suspension movement, it's referred to as bumpsteer. We generally want to minimize overall bumpsteer, as it can create directional instability when a car traverses bumps (or during a wheels-up launch). However, it can be used in moderation to create certain handling characteristics. For example, a front suspension that toes-out in bump and toes-in in droop can help a car understeer in corners.
  • Toe: Toe is the alignment measurement of a wheel relative to the longitudinal axis of a car. Toe-in refers to when the front of the wheel pointing in toward the car's centerline, and toe-out is when the front of the wheel points away from the centerline. Zero toe means the wheels are pointing straight ahead, and are parallel to each other.
  • Thrust Angle: If the rear wheels' toe angles aren't symmetric (e.g., one is toe-out and the other is toe-in), or the rear axle is shifted to one side of the car, then the rear wheels will produce a driving force that is not aligned with the centerline of the car. This misalignment is called a thrust angle. Even if the front end is aligned perfectly, the front wheels must be off-center for the car to drive straight. This is also called "dog tracking."

Before we started, we used a string setup to set the front and rear toe to zero, and we made sure the car didn't have a rear thrust angle. You could also have the car professionally aligned, and the front and rear toe set to zero. However you do it, this step is extremely important because improper toe settings will render all later measurements worthless.

Photo Gallery

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15 The goal is to find the spacer stack with the least amount of toe change through the suspension's movement, paying particular attention to the first inch of bump travel. Finding the right combination is an iterative process, but using MM's excellent instructions, we homed in on the right spacer stack pretty quickly. We plotted the final bumpsteer curves for each side, using a spreadsheet to verify our findings. Maximum Motorsports recommends you keep bumpsteer to within 0.020 per inch of suspension travel, which we've indicated on the chart with red lines. For reference, we plotted the front rightside bumpsteer curve with the OEM tie-rod end. It's surprisingly close to the best we could muster. We concluded that on this particular car (and perhaps most '96-'04 Mustangs) a front bumpsteer kit wasn't required when modestly lowering the suspension!

Photo Gallery

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27 Here's a plot of our four bumpsteer curves. Maximum Motorsports recommends you keep bumpsteer to within 0.020-inch per inch of suspension travel, which we've indicated on the chart with red lines. According to Maximum Motorsports' Jack Hidley, "While minimizing overall bumpsteer is important, it's also important to get the curves from each side to match as closely as possible. That way the car doesn't change direction when the chassis pitches."